I am interested in building my own yagi. I have been looking at various topics & software. So often, I see a number of parallel lines and some specifications. Three details are never directly stated, so I am assuming that there is a "standard" yagi...

How is the transmission line connected to the driven element? I am guessing that the driven elements would be split? Is a balun implied?

Is the beam metalic? Are the directors / reflector connected electrically to the beam?

Most yagi's use whats called a gamma match on the driven element to tune the antenna to resonance. the best thing to do is pick you up a copy of " The Radio Amatuers Handbook " which will have all the info you need on building antennas as well as other equipment. You can usually get one off of ebay for a few bucks. Also check out the ARRL website. www.arrl.gov

There is no such thing as a "standard yagi", and there isno "normal" answer to any of your questions.

For yagis, and any antennas for that matter, you need tothink of the 4 basic pieces of the antenna:

1) the electrical design (the length and spacing of theelements/wires/radiators.) For yagis, this includes thediameter of the elements, and also requires a correctionfor using a metal boom (depending on how the elements areattached.) This determines the gain, F/B ratio, etc.

2) The design frequency. You can take the same design andscale it to other frequencies, and it really is still thesame antenna, even though the dimensions have changed.

3) The feed method. You can use any of a number ofdifferent feed methods on the same antenna. Some mayrequire that the driven element be split, others don't.

4) The actual mechanical construction of the antenna.This includes details such as the boom material, howthe elements are attached to the boom, the element material(tubing? solid rod? wire? copper? aluminium?) Thisusually has only a small effect on performance (if youuse lossy materials), but makes a big difference in howlong the antenna will survive the weather.

Although there are some interactions among these fourcharacteristics (scaling an antenna to a lower frequencychanges the mechanical considerations, for example), to alarge extent a builder can take liberties in one areawithout changing the others.

My usual method for building a yagi is to start by choosingthe properties I want: generally this means reasonablegain, impedance of 20 ohms or higher, and all rear lobesof the pattern down at least 20dB. In some cases, thephysical length is the limiting factor. Then I lookthrough various designs to find one that looks good.

The next step is to decide how to build it. My favoritemethod is to stick #8 aluminum ground wire (from RadioShack) through holes in thin-wall PVC pipe, but forDF use in the woods I use tape measure material for theelements instead. (It bends when it hits a branch, thensnaps back into place.) For more permanent installationsI might use heavy aluminum tubing for the boom and3/8" rod for the elements (with or without insulatingspacers between the elements and the boom.) This alldepends on the intended application and how much time andmoney I am willing to put into the antenna.

Then comes the question of how to feed it. For mechanicalrigidity in short elements, having a continuous elementis an advantage. I have a couple of UHF yagis that Imade with a split driven element (440 and 730 MHz), andeven a good pile of hot-melt glue at the feedpoint doesn't take all of the wobble out of the driven elements.On the other hand, where there is adequate mechanicalstructure to support the elements, splitting the drivenelement and using a beta match is probably the easiestapproach. (Also, the W4RNL "OWA" antennas have direct50 ohm feedpoints, so don't need a hairpin for matching.)So my tape measure antennas tend to have split feedpoints,since the elements are firmly attached to the PVC pipeframework. Wire yagis are usually not split, and usea delta or "T" match, or the "half-folded driven element"approach shown in the "Cheap UHF Yagis" web site listedbelow. This latter method is quite simple and has becomemy favorite for many uses.

Using an insulated boom simplifies the calculation ofelement lengths. If you use a metal boom, then you haveto correct the element lengths by some amount thatdepends on just how the element is mounted with respectto the boom.

Here are some links for more information:

www.cebik.com/radio.html W4RNL's antenna site - one ofthe best antenna resources on the web. Start with his article, "I Want to Build a 3-Element Yagi". I uses thedesigns he lists here on several bands, even though theyare originally given for 10m.

BYU has it right. I have built a number of Yagi's, following the basic formula for the D.E., making the Director 5% shorter, the Reflector 5% longer...spaced 0.15 wave on the boom. They all exhibited Front to Back. Without instrumentation it is only a guess HOW MUCH. Recently I built a wire, inverted vee, fixed, 30 meter Yagi, aimed at Eu..on faith, and it worked great. I got better reports than on my dipole. I only really 'tuned' a Ten Meter one, ONCE. Man, what a job. By tuning, I mean tuning the parasites for Front to Back... Feed point impedance was ignored as I used tuned feeders. Made a "Quad Man" out of me... they are a snap to tune. [but a 'puppy' to install and keep up!]

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